At number one must be the 'god' virus... Spanish flu killed only 50 million. But more deadly and infectious than any other in history, faith in various sky pixies must have claimed many times more. Those already infected are usually incurable. They even argue that the Corona virus is valid evidence that God's exist!... a warning that we are not praying enough or something like that. Such are the demented minds affected by religion. Survivors of the god virus are strongly immunised. But having said that...gods work in mysterious ways apparently!